Using the measuring sticks I made for 3000 miles, it seems like each of the boxes formed by longitude and latitude on AM's map is about 315 miles long by 240 miles wide. At its widest, Fort Hold Territory does seem to be about 315 miles long, and at its longest (the edge of Gar to its border with Ruatha on the Eastern-facing shoreline of Big Bay) about 400 miles tall.

Wolfram Alpha says this is 126,000 miles squared, a little smaller than Germany. At the lowest setting in the Domesday Book calculator, this area would have a population of a little over three million. There would be 65,000+ without permanent residence (Holdless? Drudges?). 2.9+ million of the residents of Fort Hold Territory would live in one of about 6,500 villages, ranging in size from 30 residents up to 300.

A little over 200 thousand men and women would be living in up to 40 "towns" - here I would suggest that these represent minor holdings like Gar. There will be 7 large cities, perhaps representing the largest of Fort's beholden Minor Holds like the Seahold, with around 82,000+ folk living within those walls for a population of 12,000 people.

Fort Hold itself would have a population of 16,000+. The Domesday Book doesn't provide for a University, but we know Fort has two! I would recommend assuming that Fort Hold doesn't count Harper and Healer among its residents for tithe purposes and that the population of Fort is indeed around 10,000, with Healer and Harper splitting the 6,000 between them. (Domesday also accounts that our Fort would have 400 clergy! Pern doesn't have clergy so if you'd prefer a large population Hold and a small population Harper/Healer, you could use this number!)